The
Earthquake of 12 November 1999 in Turkey
George
Pararas-Carayannis
Introduction
On November 12, 1999,
another devastating earthquake with magnitude 7.2 struck a hilly
region of northwestern Turkey spreading death and destruction
to Duze, a city of 80,000, the city of Bolu and surrounding villages.
This was the second strongest earthquake to strike Northern Turkey
in less than three months, since the powerful 7.8 magnitude quake
of August 17, 1999 struck the densely populated area near Izmit
and killed more than 17,000 people. The November 12, 1999 earthquake
was not an aftershock but a different seismic event, apparently
along the same western segment of the North Anatolian Fault system.
It was felt widely throughout the region, in Istanbul, Ankara
and the Eastern Mediterranean coastline.
Earthquake Epicenter,
Origin Time, Magnitude and Aftershocks
The epicenter of the
November 12, 1999 earthquake was at 40.8 N , 31.6 E (Strasburg
Geodynamic Institute) in the Duze region, near the city of Bolu
, which is about 170 Km southeast of Istanbul and approximately
45 miles east of the region hit by the August earthquake. Its
magnitude was estimated at 7.2 on the Richter scale. The Strasburg
Geodynamic Institute gave the magnitude as 7.3.
Within a two hour
period following the main quake, 69 aftershocks were recorded,
five of which had magnitudes greater than 5. Of these, 30 aftershocks
had magnitudes ranging from 3-3.9, 33 had magnitudes ranging
from 4-4.9, and 6 ranging from 4.9-5.4 (Kandilli Observatory
and Research Institute). Aftershocks continued for weeks and
months.
 Geological Instability of the
regio
Modified After Nafi
Toksoz of MIT/ERL
The excessive seismicity
of this particular region has been described previously in the
report of the August 17, 1999 earthquake. Briefly restated, the
North Anatolian fault is a major fracture that transverses the
Northern part of Asia Minor and marks the boundary between the
Anatolian tectonic plate and the larger Eurasian continental
block. Because of this unstable tectonic system, this area is
considered as one of the most seismically active zones of the
world.
Tectonic and Geologic
Setting
The North Anatolian
Fault Zone is the most prominent active fault system in Turkey
along which numerous large earthquakes have occurred throughout
history. It extends for 1200 km in an approximate east west direction.
It has resulted from movements of the Eurasian tectonic plate
grinding against the Anatolian tectonic plate. At least since
1930, when instrumental measurements begun in the area, the Turkish
portion of the Anatolian plate is pushing at the rate of about
3 cm/year against the Greek portion of the plate in the vicinity
of the Northern Aegean sea near the island of Samothrace. In
fact, earthquakes in the Northern Aegean followed the August
17, Turkish earthquake, releasing the strain that had accumulated.
The net tectonic movements suggest also a rotation of the Turkish
portion of the tectonic plate in a counterclockwise direction.
As with the August
17 event, the quake of November 12, 1999 occurred along the long,
east-west trending, great North Anatolian fault zone (NAFZ) in
Northwestern Turkey. As we reported, NAFZ has many similarities
to the San Andreas fault system in California. Like the San Andreas,
earthquakes along the Northern Anatolian Fault involve primarily
horizontal ground motions.
Recent Historical
Earthquake Activity Along the Northern Anatolian Fault
Source: Kandilli Observatory and Research Institute (modified)
The earthquake of
November 12, 1999 occurred on the northern branch of NAFZ and
broke a 60-80 km segment between Duze and Bolu. This happened
also along a section of a previously identified seismic gap where
an earthquake had been expected, particularly because of the
additional strain that the August 17 event had added. Following
the August 17 quake, there had been speculation as to whether
sufficient stress had been added to either east or west segments
at either end of its rupture along the North Anatolian fault
and which of these adjacent segments would rupture next. The
November 12, 1999 quake demonstrated the rupture and horizontal
movement, this time, was toward the east.
Progressive
failure on the North Anatolian fault since 1939 (Stein et al.
1996)
Earthquake's Surface
Rupture and Ground Displacements
The duration of ground
motions from the November quake were a great deal shorter than
those of the August quake and lasted somewhere between 15 to
20 seconds. Although field surveys have not been completed, ground
displacements are estimated to cover an area of about 60-80 km
in length along the fault. The extent of horizontal ground displacements
are not known yet, nor whether they were any vertical movements.
A single vertical displacement of about 1.5 m was reported in
the Bolu area but his may have been an anomaly, perhaps the result
of liquefaction or some surface effect, as most of the movements
along the NAFZ are primarily horizontal. However, what is known
with certainty is that the direction of seismic activity had
moved in an eastward direction, for this event.
Did the November 12,
1999 Earthquake occur on the NAFZ or on a parallel fault?
The NAFZ is a very
large and well known system but there has been speculation that
perhaps this latest November 12, 1999 quake occurred on a parallel
branch to the major fault. Field surveys and studies of aftershock
epicenter and hypocenter distributions will probably shed light
on whether this latest earthquake occurred on a branch of the
NAFZ or not. However, it would seem that the August earthquake
would have relaxed the strain on parallel to the NAFZ faults.
Such relaxation of parallel faults occurred along the northern
portion of the San Andreas fault system following the Great San
Francisco 1906 earthquake, while the strain was increased on
end-to-end faults. Therefore, based on this past observation
of seismic activity release in a different area of the world,
we may conclude tentatively that the latest November earthquake
in Turkey must have occurred on an end-to-end adjacent segment
of the same NAFZ fault, which had been strained by the August
quake.
In the report of the
August 17, 1999 earthquake we provided the historical background
of seismic activity in the 20th century for western Turkey along
the Northern Anatolian Fault. We indicated that, beginning in
1939, there have been numerous large earthquakes with Richter
magnitudes of over 6.7. These have struck in progression along
adjacent segments of the the NAFZ fault. The August 17, 1999
earthquake was the eleventh in a series of such events occurring
in progression. The most recent November 12, 1999 quake was the
twelfth.
Did the
November 12, 1999 Earthquake Occur Along a Known Seismic Gap?
Starting with the
1939 event, twelve earthquakes have now broken segments of the
Northern Anatolian Fault fault in both eastward and westward
direction. Our review of historical seismic data shows that between
1939 and 1944 there was an active westward trend in the seismic
activity with a resulting surface rupture of 600 km of adjacent
fault. Subsequently, the westward trend of earthquakes slowed
down. Earthquakes occurring in 1957 and 1967 ruptured an additional
adjacent 100 km of fault but there was separated activity further
west during 1963 and 1964. There was a long seismic gap separating
the 1967 quake and the 1963 and 1964 quakes. The August 17, 1999
quake predictably occurred along a portion of this segment. Similarly,
the November 12, 1999 quake occurred along the previously unbroken
segment of this gap.
Was All
the Strain on the Seismic Gap Released by the November 12, 1999
Earthquake?
The westernmost end
of the segment that ruptured by the November quake appears to
end near the eastern most end of the segment that ruptured by
the quake in August. This would indicate that the gap for this
particular region has closed and that most of the seismic strain
was released by the November 12, 1999 quake . However, it is
not known with certainty at this time whether the closure of
the gap has been complete. There is a possibility that still
a portion to the west of the present rupture, in the area between
Sapantza and Akgiazi, a small gap may still exist and that strain
may have accumulated. Another seismic gap also remains under
the Sea of Marmara, south of Istanbul, where an earthquake and
even a small tsunami may occur in the future.
It is quite possible
that all the seismic strain was not released by the November
event and that some additional future seismic event will release
the remaining strain. A moderate size earthquake would be expected
to release the remainder of the stress in the area, but such
an earthquake may not necessarily happen soon.
 
Damage from
the earthquake
Death Toll
and Extent of Damage
As of November 18,
1999, the death toll was 619 but expected to continue to rise
because this is a densely populated area. More than 3,000 people
were reported injured and several hundreds more were missing.
The November 12, 1999 earthquake collapsed buildings and mosques
and tore apart highways. Unconfirmed reports estimate damage
at around $10 billion. There was extensive damage throughout
the area. Many buildings collapsed at the city of Bolu , near
the earthquake's epicenter, and in Smyrna some distance away.
There was extensive damage at the village of Kayanasli.,15 km
from Duze, on the way to Bolu.
Will there
be earthquakes elsewhere in Turkey?
Turkey is located
on a very seismic area of the globe where earthquakes occur with
certain frequency. The present trend of seismicity indicates
future activity to move in a western direction. We indicated
that small seismic gaps exist in the area west from where the
November earthquake ruptured the surface. There are also some
secondary fault systems where moderate earthquakes are possible
in the future. One small seismic gap, approximately 150 km long,
exists presently under the Sea of Marmara about 25 km south of
Istanbul. A moderate to large earthquake could occur along this
gap in the future and a tsunami could also be generated. There
is no way to predict at the present time when this may exactly
happen. Scientists believe that such an earthquake could occur
at any time in the next 50 years.
Conclusions
The November 12, 199
earthquake was a different major earthquake and not an aftershock
of the August 17, 1999 earthquake to the east. In all probability
it occurred along an end-to-end segment of the same fault system
rather than on a parallel collateral fault. Aftershocks are expected
to continue in the immediate area and some may be larger in magnitude.
There is no reason for an earthquake to occur in Greece as a
result of this November earthquake and the September earthquake
in Central Greece was unrelated. However, the northern Aegean
could experience some additional earthquake activity, as in the
recent past. A number of grabbens, fault offsets and other structural
topomorhological features at the bottom of the Sea of Marmara
indicate that seismic activity and movements of branches of the
North Anatolian fault extend under the sea. A seismic gap has
been identified in the area south of Istanbul where an earthquake
can be expected in the next 50 years. However, this does not
mean that it cannot happen sooner or at any time.
References
Ross S. Stein, Aykut
A. Barka and James H Dieterich, Progressive failure on the North Anatolian fault
since 1939 by earthquake stress triggering, Geophysical Journal International, Vol. 128,
594-604, 1996.
M. Nafi Toksoz, A.F. Shakal, and Andrew J. Michael, Space-time migration of earthquakes
along the North Anatolian fault zone and seismic gap, Pageoph, Vol. 117, 1258-1270,
1979.
Earthquake Information from National Earthquake Information Center
(NEIC) (USGS)
Earthquake Information from Bogazici University, Kandilli Observatory
and Research Institute.
Special page on the earthquake by IRIS (Incorporated Research
Institutions for Seismology)
U. Kuran and A.C.Yalciner (1993), Crack Propagations, Earthquakes and Tsunamis
in the Vicinity of Anatolia,
in Tsunamis in the World, ed.by S.Tinti, pp.159-175
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